Acoustic source characterisation for non-linear in-duct sources

Sammanfattning: Different ways to model acoustic in-duct sources have beenanalysed. Measurements were conducted on two diesel truckengines. It was found that for steady state oscillations, onecould linearise in the neighbourhood of a given linearisationpoint, i.e. engine speed, engine load and acoustic load. A morenon-linear source gave a smaller neighbourhood where thelinearisation was valid in comparison to the correspondingneighbourhood of amore linear source.Two different linearity tests have been proposed andanalysed. These tests are to be applied to systems where onlyoutput data is available. A method to increase the sensitivityof the linearity measure was also suggested.In order to model an acoustic in-duct source, one has tochoose if the model should be linear, non-linear or hybridlinear / non-linear. Hybrid methods were tested for a simplepiston-restriction system with satisfying results.Several of the hybrid methods were then tested and comparedto each other, quantitatively and qualitatively. It was foundthat the harmonic balance method, which is a steady statemethod, and the convolution method with a reflection functionwere the best for the present application. The calculationsconverged toward a solution for both methods in almost allcases. They furthermore gave similar results for the same testcase in the comparison.The results from the harmonic balance method were finallycompared to measurements. The accuracy was good in some cases,but worse in otherA general conclusion from the results presented in thisthesis, would be that it is necessary to use non-linear sourcemodels in several cases. The receiving system could in mostapplications though be kept in the linear frequency domain forpredictions within engineering accuracy. The HBM and theconvolution method provide the coupling in the differentsituations that arise during design processes. Finally, amethodology in how an acoustic in-duct source can be analysedwas presented.Keywords:in-duct source, source characterisation,non-linear source, linearity test, hybrid method, IC-engine,muffler, coupling method